This invention relates to a belt transmission such as an automatic belt transmission for motorcycles or other vehicles, and more particularly to cooling arrangements for cooling the component parts of such a power transmission.
In motorcycles equipped with small output engines where small torque is transmitted to the driving wheel, a drive belt, which is made of rubber as a base material, is used to transmit rotation of the crankshaft of the engine to the driving wheel. An automatic belt transmission, which uses such drive belt, is generally employed to change the engine speed. A typical automatic belt transmission is adapted to steplessly change the speed ratio between a drive pulley on the crankshaft side and a driven pulley on the driving wheel side in such a manner that the drive pulley has its pitch diameter automatically changed due to centrifugal force which is generated by rotation of the crankshaft, and the change of the pitch diameter causes a corresponding change in the tension of a belt wound on the pulleys, which in turn causes a change in the pitch diameter of the driven pulley.
The above automatic belt transmission is rather simple in construction and can nevertheless perform transmission in a stepless manner. Therefore, it is very advantageous to apply such automatic belt transmissions to vehicles such as motorcycles, which are required to be compact in size and inexpensive, and also have a wide working speed range.
However, according to the above type automatic belt transmission, the efficiency of transmission of power from the engine to the driving wheel is determined by the product of the coefficient of friction of the belt and the urging force of the belt against the pulleys. Consequently, if the urging force of the belt against the pulleys is set to a large value in order to obtain large power transmission efficiency, a great deal of friction heat can be generated at the contacting portions of the belt and the pulleys, to cause overheating of the belt. Thus, the automatic belt transmission has limited maximum power transmission efficiency, which imposes substantial limitations upon the output and total stroke volume of an engine to be installed on a vehicle to which the automatic belt transmission is applied.
Furthermore, the transmission mechanism is accommodated within an enclosed casing in a manner isolated from the outside, so as to avoid intrusion of dust, mud, water, etc. into the casing for protection of the transmission mechanism. This arrangement can cause a rise in the temperature inside the casing in cooperation with the generated friction heat to promote the overheating of the belt, which can result in a shortened service life of the rubber base belt.